Liquid crystal displays have found widespread usage in the prior art, one such display being, for example, of the twisted nematic liquid crystal type. Such LCDs operate by applying an alternating voltage potential between opposing electrodes sandwiching a liquid crystal layer.
Twisted nematic displays used, for example, in cockpits of air vehicles typically include a matrix array of liquid crystal picture elements (i.e. pixels) and a corresponding backlight for illuminating the elements. These pixels are often temperature-dependent with respect to their normal operating characteristics in that an LCD relies on the behavioral characteristics of its twisted nematic crystalline layer as it is exposed to driving voltages. When such driving voltages are applied across the LC material, the nematic liquid crystals tend to align themselves so as to provide a desired image to the viewer. Because such voltage-related behavior of the twisted nematic material is a function of temperature, the overall performance of the corresponding display is temperature dependent. When below a predetermined temperature, twisted nematic liquid crystal material does not behave in a consistent manner. Accordingly, the LC material in such situations must be heated to the aforesaid predetermined temperature in order to achieve satisfactory functionality. The amount of time it takes for the LC material to be heated to this level is known as the display's "warm-up" period or time.
It is known to provide liquid crystal displays with heaters. U.S. Pat. Nos. 4,643,525, 4,723,835, and 5,247,374 are typical examples of such liquid crystal display heaters, just to name a few.
LCD heaters including a conductive ITO coating deposited on a transparent glass substrate are old and well-known in the art. In such heaters, the ITO coating is typically energized by way of a pair of parallel buss bars aligned along a planar surface of the ITO. Heretofore, such buss bars have been made of either silk-screened conductive epoxy or deposited metal, the deposited metal including, for example, a tri-layer combination of chrome, nickel, and gold. Such conventional LCD heaters and corresponding buss bars have several drawbacks discussed as follows.
Conductive epoxy buss bars which are silk-screened onto the ITO heater layer experience less than desirable conductance. This lack of conductance associated with typical epoxy buss bars often leads to a more lengthy than desired required time period for warming up corresponding displays. In other words, the use of epoxy buss bars often necessitates an increased display warm-up period.
A display's warm-up period is very important. For example, the warm-up period/time of a cockpit mounted display in a military jet aircraft sitting on an Alaskan runway on alert dictates the time the pilot must wait before taking off. Accordingly, the smaller the requisite warm-up period, the better.
Furthermore, in order to interface such prior art epoxy buss bars to a corresponding voltage source, a conductive wire is typically provided, one end of the wire being adhered to the epoxy buss bar and the other end conductively attached to the voltage source. The process of conductively attaching the wire to both the epoxy buss bar on the ITO surface and the corresponding voltage source is time consuming, messy, and often unreliable. Additionally, the silk-screening of such epoxy buss bars is also an expensive and time consuming process.
The use of deposited metal (e.g. chrome, nickel, gold tri-layer combination) as a buss bar on an ITO layer also has a number of disadvantages associated therewith. First of all, the deposition of the metal onto the ITO layer is difficult, expensive, and time consuming. Secondly, in order to interface the deposited metal buss bar with a corresponding voltage source, a wire is typically soldered onto the metal buss bar. This soldering process is difficult and fragile in that if the solder is overheated, the result is a burned out buss bar.
Another drawback of the aforesaid prior art buss bars is the thickness of the electrical connection points interfacing the buss bars and corresponding electrical supply (i.e. how far such connections protrude from the planar ITO surface). These protrusions are stress concentrators if sandwiched between plates, and pose significant design problems. Because of the relatively thick nature of such connections, the transparent glass substrate supporting the ITO layer is typically adhered directly to the exterior surface of a corresponding display's rear polarizer, thereby positioning the supporting glass substrate between the ITO heater layer and the display itself. This distancing of the heater layer from the liquid crystal material unfortunately insulates the interior of the display from the warmth or heat generated by the ITO, necessarily increasing the display's warm-up time. Furthermore, because in the prior art the glass is between the ITO layer and the display's rear polarizer, the ITO is exposed to the outer environment, thus leading to much of the ITO generated heat escaping into the adjacent atmosphere thus resulting in an increased warm-up period.
Another problem associated with having the ITO layer exposed to the atmosphere is the possibility of the ITO being scratched. If this occurs, the entire LCD cell must immediately be replaced.
As will be appreciated by those of skill in the art, the aforesaid prior art provisions of heaters within liquid crystal displays are difficult and costly processes which do not leave much room for error.
It is apparent from the above that there exists a need in the art for an LCD heater including a conductive heating layer (e.g. ITO layer) disposed adjacent the rear display polarizer so as to decrease the display's warm-up period and reduce the possibility of the ITO being scratched. There also exists a need in the art for a pair of relatively thin and easy to install and electrically connect buss bars for energizing the heating conductive layer, the buss bars being deposited substantially in parallel to one another on the conductive layer.